Vehicle internal combustion engines are capable of operating over a wide speed range. The range varies from a few hundred to several thousand revolutions per minute. Required engine accessories such as alternators, compressors for air conditioners and air brakes, pumps for steering systems, suspension systems, and control systems, and other accessories are most efficient if operated over a speed range that is narrower than the speed range of internal combustion engines. The problem is solved on some industrial vehicles by running the internal combustion engine at constant speed. Constant engine speed operation is only practical in vehicles that operate under a substantial load all or most of the time. A majority of vehicles operate under varying load requirements and require engines that operate over a wide speed range. Vehicle accessories are designed to provide a required output over a selected speed range. The size of any given accessory generally depends on the required output and the operating speed range. The required output is often fixed. The size and weight of accessories is therefore a function of the operating speed range.
Increasing the operating speed of internal combustion engine accessories makes it possible to design accessories that are smaller in overall size and have less weight. The size and weight of engine accessories is a critical factor in efforts to reduce vehicle weight and to improve vehicle efficiency.
Two-speed alternator drives are known. U.S. Pat. No. 4,862,770 to Smith discloses a two-speed alternator drive which works well. The Smith patent is owned by the assignee of this invention. The metal gears employed in known two-speed engine accessory drives are expensive to manufacture, heavy, and can be noisy. All of these problems are magnified as operating speeds increase.